A MiR-20a/MAPK1/c-Myc Regulatory Feedback Circuit In Control Of Breast Cancer Cells Proliferation And Chemoresistance

Breast cancer,with leading incidence rate and fatality rate among all cancers,has become one of the most common female malignant adenocarcinomas in theworld.Chemotherapies are major treatments for cancers.However,with the progress of treatment,patients often become resistant to chemotherapy and result in treatment failure.Therefore,the drug resistance of cancer cells is a major obstacle to the successful treatment of cancer.However,the molecular mechanism of chemoresistance remains unclear.Previous studies have implied that microRNAs(miRNA)may involve in the progress of tumor cell resistance by post-transcriptional regulation.Thus,through studying the role of key miRNAs in chemoresistance,we may able to deepen understanding of breast cancer drug resistance and lay the foundation for miRNA as a way to treat breast cancer.In the present study,we used medullary breast cancer cell line BCap37 as a model to identify and study the miRNAs that can regulate its tolerance to paclitaxel(PTX).Twochemoresistant cell lines were obtained by long-term screening from BCap37 cells in our laboratory.By miRNA microarray and high-content screening system,we identified that miR-20a-5p and miR-20b-5p had different expression pattern between parental cell BCap37 and two chemoresistant cell lines.MiR-20a-5p and miR-20b-5p belong to the same miRNA family,their roles in cancer resistance and underlay mechanism still unclear.This study demonstrates the role of the two miRNAs,especially miR-20a-5p,in chemoresistance.Besides we comprehensively study in the dysregulated expression of miR-20a-5p in breast cancer cells and its role in chemoresistant regulation.The cell experiment reveals that miR-20a-5p and miR-20b-5p effectively inhibited the proliferation and chemoresistance of not only BCap37 and related breast cancer cells but also that of cervical carcinoma cell line HeLa.Moreover,tested miRNAs also reduce the resistance of breast cancer cells to other chemotherapeutic drugs such as VCR,DOX,GEM,5-Fu in breast cancer cells.The following in vivo experiment results indicate that miR-20a inhibit tumor growth and enhance the anti-tumor effect of PTX.We predicted the possible target genes of miR-20a by bioinformatics technology and then verified function of then by luciferase reporter assay.The results indicate that TGFBR2、PDGFRA、CCND1、E2F1 and MAPK1 may act as the target genes of the miR-20a family in breast cancer proliferation and drug resistance.Among these five genes,the luciferase activity of MAPK1 was reduced most significantly and the function of MAPK1 has not been reported as target gene of miR-20a in breast cancer.The following study demonstrates that miR-20a inhibit cell proliferation and drug-resistance by directly targeting MAPK1.On one hand,MiR-20ainhibited cell proliferation and induced cell apoptosis by silencing the expression of MAPK1 and the following MAPK/ERK signaling pathway.On the other hand,the inhibitory of MAPK1 following down regulate the expression of P-gp,thus reduced the drug resistance of cancer cells.The present study then demonstrates the mechanism dysregulated expression of miR-20a-5p in breast cancer cells.The histone methylation abnormalitiesgene site may act as one of the mechanisms for the downregulation of miR-20a.Besides,c-Myc,the important factor down-stream MAPK/ERK pathway,promotes miR-20a expression and forms a feedback loop.Thus,the progression of the tumor ultimately depends on the ability of MAPK1 and c-Myc to escape the inhibition of miR-20a.In conclusion,the chemoresistantce related miRNA,miR-20a and miR-20b,may act as clinical predictive factors in breast cancer.They effectively inhibit cancer cell proliferation and chemoresistance by directly targeting MAPK1.Our study also reveals the presence of miR-20a/MAPKl/c-Myc signaling loop in breast cancer,which may represent a novel mechanism for breast cancer growth regulation.